This invention relates to bone screws; that is, those surgical fastenings by which two pieces of bone may be held together. Although usable in many forms of bone connection, bone screws of the kind under consideration, while not restricted thereto, are particularly useful in cross fixation of fractured or severed bone fragments.
In almost all bone connections of the kind under consideration, it is essential for the fractured surfaces to be united to be brought into closely contacting mutual confrontation. This intimacy of contact is usually referred to as "compression". The actual need is for the fractured surfaces to be in close, well-fitting contact and to be so held during the healing process. In practice, the simplest way of ensuring this close contact is, where practicable, to apply a compressive loading to the bone portions in a direction substantially normal to the fracture faces.
Hitherto, standard bone screws have been of two typical forms. One of these has a thread only at its leading end, the head at the trailing end being separated from the thread by a smooth, cylindrical shank. It will be clear that such a bone screw, by threading wholly in the remote bone fragment and extending freely through the near fragment, can provide compressive action upon the fractured faces to be united.
The second type of bone screw has a stem or shaft threaded over its full length. Such a screw can only be used to apply compression between two bone fragments if the near fragment is "over drilled" so that the thread engages solely in the remote fragment, the near fragment being free to move over the stem of the screw during insertion.
In both these cases it will be clear that this gaining of compressive action is achieved at the expense of subjecting the bone surface immediately below the screw head to concentrated bearing loads, and also at the expense of the screw head constituting a relatively large protrusive element.
It is not uncommon for the standard type of bone screw to be used with washers or other metallic inserts between the screw head and the adjacent bone surface. Any such inclusions are likely to provide some degree of load spreading which would relieve bearing stress intensity on the bone surface. But this load spreading facility is often insufficient because bone surfaces are very rarely flat and even where a washer or other insert is provided (unless speciallymoulded closely to fit the bone), the bearing pressure exerted by the screw head remains, in effect, a highly concentrated load and crumbling or other failure of the adjacent bone affected frequently occurs without sufficient compressive action at the fractured faces being achieved.
The standard type of bone screw has several other shortcomings. For example, the near bone fragment is held relative to the remote portion purely by compressive forces which act in the line of the screw; the screw head merely abutting the cortical surface of the near fragment as distinct from being firmly and securely anchored to that fragment. If the cortical bone layer directly under the screw head provides inadequate support either during insertion or subsequently, then compression is lost and the near bone fragment is free to move relative to the trailing part of the screw. In consequence, relative movement between the bone portions to be held can occur.
A further adverse factor is that of bone resorption. This may be a physiological response to localized pressure and may occur either directly under the screw head or at the fracture site. Clearly, where such resorption occurs, loosening of the screw may also arise, with subsequent movement of the bone fragments.